1. Field of the Invention
The present invention relates to a process for producing a vinyl chloride polymer having a large bulk specific gravity.
2. Description of the Prior Art
Vinyl chloride polymers are resins excellent in physical properties and are broadly employed as a rigid polyvinyl chloride or a flexible polyvinyl chloride. In recent years, there are demanded a vinyl chloride polymer having a large bulk specific gravity capable of increasing an extrusion rate in extruders in order to improve the productivity of polyvinyl chloride products when extrusion molding rigid polyvinyl chlorides. Conventionally known suspension polymerization methods for the purpose of obtaining a vinyl chloride polymer having a large bulk specific gravity include, for example, a method charging an additional monomer during polymerization see Japanese Pre-examination Patent Publication (Kokai) No. 59-168008!, a method using a highly saponified polyvinyl alcohol as a suspending agent see Japanese Pre-examination Patent Publication (Kokai) Nos. 57-7600 and 4-323204, and Japanese Patent Publication (Kokoku) No. 1-27088! and a method charging an additional suspending agent during polymerization see Japanese Pre-examination Patent Publication (Kokai) Nos. 3-70703 and 5-39309!. These methods can increase the bulk specific gravity of the resulting polymer to some extent, while to the contrary, they have the problems that the gelation rate of the polymer is slow and fish eyes increase in formed parts obtained from the polymer. Further, especially when a so-called nontoxic stabilizer such as Ca-Zn stabilizer is added to a compound, in which various additives have been compounded in the resulting polymer, for use in producing formed parts, there are problems in that the bulk specific gravity of the compound is not so large and also the extrusion rate of the compound in extruders does not increase. Furthermore, since such a nontoxic stabilizer is weak in stabilizing action for polymers, there are also problems in that products (formed parts) from the resulting polymer are unsatisfactory in thermal stability, initial coloration, etc.